Low temperature P2 O5 oxide diffusion source

ABSTRACT

The present invention relates to a solid low temperature phosphorus diffusion source that is an R 2  O 3  /P 2  O 5  compound in which the ratio of R 2  O 3  to P 2  O 5  is 1 to 5 and R is Nd, Eu, Pr, Sm, Ho, Tb, Er, Yb, Tm or Dy. The invention also relates to a method of making the diffusion source, a method of using the diffusion source to evolve P 2  O 5  to dope a silicon wafer, and the doped silicon wafer.

The present invention relates to a solid low temperature phosphorus diffusion source that is an R₂ O₃ /P₂ O₅ compound in which the ratio of R₂ O₃ to P₂ O₅ is 1 to 5 and R is Nd, Eu, Pr, Sm, Ho, Tb, Er, Yb, Tm or Dy. The invention also relates to a method of making the diffusion source, a method of using the diffusion source to evolve P₂ O₅ to dope a silicon wafer, and the doped silicon wafer.

BACKGROUND OF THE INVENTION

A solid diffusion source comprising glass ceramic or polycrystalline particles is disclosed in the Rapp U.S. Pat. No. 4,141,738. In U.S. Pat. No. 4,033,790 there is disclosed solid diffusion sources made by hot pressing R₂ O₃ /P₂ O₅ (La₂ O₃ /P₂ O₅, Ce₂ O₃ /P₂ O₅ and Y₂ O₃ /P₂ O₅) compounds in which the R₂ O₃ to P₂ O₅ is mainly 1 to 5. The above U.S. Patents are incorporated by reference.

U.S. Pat. No. 4,846,902 discloses a solid diffusion source that is a Gd oxide/P₂ O₅ compound in which the ratio of Gd oxide to P₂ O₅ to about 1/1 to 1/5.

It is desirable to provide a solid diffusion source that is easily and efficiently made, and a source that evolves P₂ O₅ relatively rapidly at a relatively low temperature.

OBJECTS OF THE INVENTION

It is an object of the present invention to prepare a doping composition as a diffusion source, to provide a doped silicon wafer, the composition being a penta phosphate R₂ O₃ /P₂ O₅ in which the ratio of R₂ O₃ to P₂ O₅ is 1 to 5 and R is Nd, Eu, Pr, Sm, Ho, Tb, Er, Yb, Tm or Dy.

It is an object of the present invention to provide a method of making a doped silicon wafer using a solid diffusion source, the method comprising the steps of:

A. heating R₂ O₃ oxide and H₃ PO₄ to provide a R₂ O₃ oxide/P₂ O₅ composition consisting essentially of a R₂ O₃ /P₂ O₅ compound in which the ratio of R₂ O₃ to P₂ O₅ is 1 to 5 and R is Nd, Eu, Pr, Sm, Ho, Tb, Er, Tm, Yb or Dy.

B. firing the composition to evolve P₂ O₅ to deposit a phosphorus containing coating on the silicon wafer at a relatively low temperature and at a relatively high rate to form a glassy layer on the silicon wafer.

These and other objects will be apparent from the specification that follows and the appended claims.

SUMMARY OF THE INVENTION

The present invention provides a composition for a solid diffusion source to deposit a phosphorus oxide coating on a silicon wafer, the composition consisting essentially of an R₂ O₃ /P₂ O₅ compound in which the ratio of R₂ O₃ to P₂ O₅ is about 1 to 5, and R is Nd, Eu, Pr, Sm, Ho, Tb, Er, Yb, Tm or Dy.

The present invention also includes a method of using the doping source by firing it at about 750° or 800° C. up to about 950° or 975° C.; and the doped silicon wafer itself.

The present invention also provides a method of making and using the doping source at a temperature preferably at about 800° to 925° C.

The present invention also provides a doping composition in which the above described penta phosphate doping compound contains ZrO₂. The present invention also provides the above-described R₂ O₃ ·5P₂ O₅ compound in which it contains ZrO₂.

DETAILED DESCRIPTION OF THE INVENTION

In the present invention, R is Group IIIa element (according to the Periodic Chart of the Elements, Page 46 of the International Critisol Tables of Numerical Data, Physics, Chemistry and Technology, 1926, McGraw-Hill). The suitable group IIIa elements have atomic numbers from 59-70 but not 64.

In one aspect, the R element has an atomic number from 59-63 (Pr, Nd, Sm and Eu). Suitable R elements also have an atomic number from 65-70 (Tb, Dy, Ho, Er, Tm and Yb) and preferably 66-68 or from 57-63 but not 61.

Preferred R elements are Yb, Dy, Er and Sm--these compounds being colorless or white or near white. Suitable R elements are Pr, Nd, Tb and Eu.

The pentaphosphate compounds evolve P₂ O₅ to provide a glassy layer of about 300 or 400 or 500A° up to 1000-2500A° generally at a temperature at about 750° to 975° C. and as high as 1025° C.

EXAMPLE 1

Various R₂ O₃ ·5P₂ O₅ sources were prepared using stoichiometric amounts of R₂ O₃ and P₂ O₅ (in the H₃ PO₄ starting material) as described in the Pickrell U.S. Pat. No. 4,846,902 for R₂ O₃ ·5P₂ O₅ materials and in U.S. Pat. No. 4,033,790 for Gd₂ O₃ ·5P₂ O₅ materials.

In each case, the R₂ O₃ and H₃ PO₄ were mixed with a excess of H₃ PO₄. The excess water is evaporated off, and the mixed product fired at 750° C. for 15 hours to provide the R₂ O₃ ·5P₂ O₅ product.

The fired material was pressed and sintered as in the U.S. Pat. No. 4,033,790, the resultant billet being sliced into individual diffusion source wafers.

In a typical formulation, the following was mixed in approximate grams to provide approximately the following mixture:

    ______________________________________                                         Ingredients         Grams                                                      ______________________________________                                         Nd.sub.2 O.sub.3    112                                                        (1 mole per 5 moles P.sub.2 O.sub.5)                                           H.sub.3 PO.sub.4 (85%)                                                                             384                                                        ______________________________________                                    

The procedure followed is set forth in the aforementioned Pickrell U.S. Pat. No. 4,846,902, the water of the R₂ O₃ /P₂ O₅ mixture being driven off the mixture by heating at 150° C. Thereafter, the crucible was covered and fired at 750° C. for 15 hours to provide the neodymium oxide/5P₂ O₅ product. The resultant crystalline product was ball-milled to obtain a fine powder (-60 mesh).

Silicon wafers and the diffusion source were heated to 900° C. and the pentaphosphate source fired at 900° C. for 1 hour to evolve P₂ O₃ and coat the heated wafers with a glassy film of about 1000A° in thickness. This is a sufficient amount of glass to satisfactorily dope the wafer at 900° C. The sheet resistively of the doped wafer was satisfactory. Excellent film thickness also was obtained by firing for 1 hour at 825° to 975° C.

Other doping source R₂ O₃ ·5P₂ O₅ compounds were prepared and fired for 1 hour to evolve P₂ O₅ at the temperature indicated in the table below to coat silicon wafers with a glassy film of about 300-2000A°.

    ______________________________________                                         R.sub.2 O.sub.3.5P.sub.2 O.sub.5 Compound                                                       Firing Temperature °C.                                 ______________________________________                                         Pr.sub.2 O.sub.3.5P.sub.2 O.sub.5                                                               850-950                                                       Nd.sub.2 O.sub.3.5P.sub.2 O.sub.5                                                               825-950                                                       Sm.sub.2 O.sub.3.5P.sub.2 O.sub.5                                                               850-950                                                       Dy.sub.2 O.sub.3.5P.sub.2 O.sub.5                                                               850-975                                                       Er.sub.2 O.sub.3.5P.sub.2 O.sub.5                                                                850-1025                                                     Er.sub.2 O.sub.3.ZrO.sub.2.5P.sub.2 O.sub.5                                                     800-975                                                       ______________________________________                                    

Er₂ O₃ ·5P₂ O₅ evolves P₂ O₅ at a good rate over a wide temperature range, glassy films of 200A° or more deposited at 800° to 1025° C.

As in the case of Er₂ O₃ ·ZrO₂ ·5P₂ O₅, substantially similar results can be obtained by the use of ZrO₂ in the other rare earth oxide/·5P₂ O₅ compounds.

In the R₂ O₃ ·5P₂ O₅ compounds, R is a Group IIIa rare earth element (International Critical Tables) having an atomic number as follows:

    ______________________________________                                         R            Atomic Number                                                     ______________________________________                                         Pr           59                                                                Nd           60                                                                Sm           62                                                                Eu           63                                                                Tb           65                                                                Dy           66                                                                Ho           67                                                                Er           68                                                                Tm (or Tu)   69                                                                Yb           70                                                                ______________________________________                                    

The above examples illustrate the R₂ O₃ ·5P₂ O₅ diffusion sources, the methods of making the diffusion sources, the methods of using the sources by firing the same to evolve P₂ O₅, and methods of making the doped silicon wafers that are useful in a typical emitter diffusion, in channel MOS diffusion and in the doping of a polysilicon layer without undesirable toxic gases and liquids. 

What is claimed is:
 1. A composition for a solid diffusion source to deposit a phosphorus oxide coating on a silicon wafer, the composition consisting essentially of an R₂ O₃ /P₂ O₅ compound in which the ratio of R₂ O₃ to P₂ O₅ is about 1:5, and R is Nd Eu, Pr, Sm, Ho, Tb, Er, Yb, Tm or Dy.
 2. A composition as defined in claim 1 in which R is Nd, Eu, or Pr.
 3. A composition as defined in claim 1 in which R is Sm, Ho, Tb or Er.
 4. A composition as defined in claim 1 in which R is Tm, Tb or Dy.
 5. A composition as defined in claim 1 in which the R₂ O₃ /P₂ O₅ compound contains ZrO₂. 